Labkotec Publishes Pre-Certificate Report for Ice Detection

PIRKKALA, September 15th, 2011.

Setting Standards For Ice Detection
Reliable operation, consistent performance, reduced maintenance and improved safety are key drivers for the wind turbine business. Consequently, Labkotec Oy decided to put its latest generation of ice detector, the LID-3300IP, through a rigorous independent testing regime to ensure that it would meet the standards of modern wind energy operators.

The need for Ice Detection Standards
Icing of wind turbines is a common issue in northern latitudes and at elevated sites, building up on rotor blades, weather sensors and power lines, for example. As well as its effect on energy production efficiency, ice build up on rotor blades can cause damage to buildings and people near the wind turbines. Efficient ice detection brings safety to arctic wind turbines. So, for operational, maintenance and safety reasons, ice detection is a desirable feature for wind turbines located in harsh environments.

Until now, there have been no set standards or certification tests to prove the performance and set best practice for ice detection. This is why Labkotec Oy decided to work with VTT Technical Research Centre of Finland, the largest multi-technological applied research organization in Northern Europe. VTT is a part of the Finnish innovation system under the domain of the Ministry of Employment and the Economy and is a non-profit-making research organization.

Development of Evaluation Methods
VTT has created in-house methods to evaluate instrumental icing. The method for icing detectors for wind turbines is based on detection time in standard icing conditions. With Labkotec Oy, it has engaged on a pre-certification process which may be a precursor to the development of formal certification rules, standards and best practices for icing or ice detection tests.

Labkotec Ice Detectors
Labkotec Ice Detectors have been specially designed for the detection of ice on the rotor blades and are capable for detecting incloud icing and freezing rain. Ice detectors are compact and easy to install and no calibration is needed making them suitable both for old and new wind turbines and are easy to connect to wind turbine control systems. Congealment status, alarms and parameters can be monitored and adjusted through an in-built web-based user interface. The unit allows for two analogue output signals so that both external temperatures and freezing conditions can be measured.

For the pre-certification, an LID/ISD ice sensor plus the LID-3300IP Ice Detector control unit was chosen. This device is based on ultrasonic signal detection where the output signal weakens in the presence of ice. When a set alarm level is reached, ice detector then gives a signal to turbine control system to either heat the rotor blades or stop the turbine operation.

Icing measurement test methods
Icing measurement tests were conducted in VTT’s icing wind tunnel which fulfils the standard icing conditions. In the tests, a clean ice detector with factory settings was exposed to the standard icing conditions and the time between the start of icing conditions and ice alarm of the ice detector was recorded. The icing measurement tests were repeated for five different ice detector units in order to verify manufacturing quality and to get statistical data from the measurements.

The Labkotec Ice Detector is typically mounted on the top of nacelle or to meteorological mast. Ice accretion calculations on wind turbine blades were conducted with TURBICE software in order to estimate the correspondence between ice detector response time and ice build-up on a wind turbine blade. TURBICE is a comprehensive numerical panel method based 2D ice accretion software.

Testing Regime
The ice detectors were tested by VTT by measuring detection time in standard icing conditions. Icing measurement tests were done in VTT’s icing wind tunnel in several phases and using acceleration by using increased rate of icing. The purpose of conducted tests and studies was to evaluate the suitability of the device for wind energy related ice detection, especially to define the response time of the ice detector, how quickly icing is detected after icing conditions have started.

The icing rate is strongly affected by numerous parameters such as: wind speed, dimensions of structure, temperature, water content and droplet size. The simulation tests were based on a 44 meter wind turbine rotor blade, and the standard ice alarm response time of the ice detector corresponds to an ice accretion on the leading edge of the blade which increases the chord length by 1,3 cm at 85 % of the maximum blade radius.

As all the possible weather condition combinations in nature cannot be covered in the test, VTT has defined standard icing conditions which are most relevant conditions to icing in wind turbine operation. Standard icing conditions are defined as: temperature -5 °C, droplet size 17 µm and wind speed 10 m/s. The droplet size has been defined based on RMC –measurements (Rotating Multi Cylinder). The standard icing conditions are representative for in-cloud icing and resulted ice type is rime ice.

Results of icing measurement tests
Based on wind tunnel tests, the device gives an icing alarm in 45 min on average when using factory settings for ice alarm level. The first indication of icing, ice detection signal amplitude decrease from 100, is noticeable after 24 minutes on average. Results between different units were very similar. The ice alarm response time can be shortened by changing the ice alarm level to be more sensitive. The results depend moderately on direction.

Based on ice accretion calculations, it can be estimated that about 18 kg of ice is built up on the blade during the ice alarm response time. Ice was distributed on the 44 meter blade so that ice build-up started at R=10m and increased according to power law curve to 1,3 cm at 85 % at maximum blade radius. The average icing thickness was 1 cm. The ice was formed only on the leading edge.

Conclusions of the testing
Thanks to the pre-certification work carried out by VTT in collaboration with Labkotec Oy, there are now an established set of tests and measurements that can be used to define ice detection performance. In its research report, VTT has stated, “The LID-3300IP Ice Detector is applicable for wind turbines and meteorological weather stations. Based on the assessment the device is capable for detecting in-cloud icing and freezing rain. The functionality of the detector is dependent on settings and the operative performance can be optimized for each weather conditions and purposed by adjusting the settings.”

Further the VTT reports notes, “The oval shape of the LID/ISD ice sensor with multiple effective “cylinder” diameters is a benefit. The collision effectiveness on structure is dependent on combinations of flow conditions; structure dimensions, droplet sizes, droplet speed etc. In different conditions the icing begins in different effective diameter. This device is capable of detecting the icing in different conditions quite quickly.”

Setting standards for the future
For Labkotec Oy, the importance of the testing regime developed by VTT is that the company is able to demonstrate that its LID-3300IP Ice Detector can meet all official requirements and building permit standards set against a defined and clear set of performance criteria. The ability to show improved turbine production reliability and radically reduced risks caused by ice formation in arctic conditions is not only an important selling point for Labkotec Oy, but sets out a clear message to other manufacturers on the need to measure up their performance in the wind energy market.